QUASI-POLYGONAL RIDGE NETWORK ASSOCIATED WITH NON-TECTONIC SAG SYNCLINES IN THE EOCENE DRUNKA FORMATION, WESTERN DESERT, EGYPT: EVIDENCE FOR HYPOGENE KARST?

Eocene limestones of the Thebes Group in the east central Western Desert of Egypt contain thousands of narrow, non-tectonic sag synclines with shallow limb dips and multiple basin closures. The synclines form an interconnected network with dominant NNW-SSE and WNW-ESE orientations, parallel to prominent regional joint sets. The syncline network occurs in limestone that is otherwise flat-lying, and intersyncline areas are broadly anticlinal only where synclines are closely spaced. We have proposed a hypogene karst model involving movement of aggressive fluids along joints and faults upward into Thebes Group limestones, causing dissolution in the lower Thebes and sag at shallower levels.

High resolution satellite imagery shows an area in the central Drunka Formation of the Thebes Group that contains a network of resistant ridges in addition to a discontinuous network of 200-400 meter-wide synclines. Ridges are 20-40 m wide, 2-5 m high, and show a strong spatial relationship to synclines. Where a syncline dies out along trend, a ridge commonly extends in the same direction beyond the syncline. Single ridges aligned parallel to the axial surface trace occur in some syncline cores, and, in other synclines, pairs of ridges flank the syncline core. Ridges form a quasi-polygonal network outlining areas 0.7-2 km across that are similar in scale and shape to intersyncline areas to the north and east. The ridges are bedrock features that display dominantly horizontal bedding (or very gently dipping bedding where ridge pairs occur along syncline limbs) that is continuous with bedding away from the ridges, suggesting the presence of quasi-vertical zones that cut across bedding in the limestone along which a process has occurred to make the limestone in those zones more resistant than the country rock. The limestone in the ridges contains multiple fracture sets and veins and is more extensively fractured than limestone away from the ridges. We suggest that synclines and ridges both formed in conjunction with hypogene fluids moving upward along a regional network of faults and fractures, with ridges forming where fluids altered the limestone and/or mineralized fractures, making the limestone more resistant, and synclines forming by sag over areas of more extensive dissolution or volume reduction.